1. Technical Field
This application relates to handling substrates, and more particularly to handling of a substrate with control over an operating range spanning a minimum of three and a maximum of six degrees of freedom.
2. Description of Related Art
Substrate handling mechanisms are often used in equipment designed to process semiconductor wafers, flat screen liquid crystal displays, printed circuit boards, and micromachine assemblies. Similar mechanisms are used in failure analysis systems.
Modern semiconductor wafers may be cylindrical substrates of silicon, up to 300 mm in diameter, and may be less than 1 mm thick. During many of the manufacturing processes of semiconductor devices, wafers are held on a substrate holder known as a chuck, using vacuum. The chuck is often also used as a substrate handler, to position the substrate at a specified location in up to six dimensions, and to move the substrate from an input section of a processing or testing system, through the process steps, and finally to an output section for removal. A chuck may be machined from aluminum, silicon carbide or other material, having a top surface that is machined to be flat. There may be vacuum outlets in the flat surface of the chuck that may be in the form of connected grooves cut into the flat surface. The vacuum system holds the wafer, or other planar substrates such as liquid crystal display panels, to the flat chuck surface. A non flat substrate or wafer may be made flatter by the action of the vacuum hold down of the chuck. On the other hand, the strain placed upon the chuck by the flattening action of the vacuum may result in warpage of the chuck surface, and consequent loss of planarity of the wafer or substrate. One method for addressing the chuck warpage problem is to make the chuck thicker and more massive, and thus more resistant to the strain of the wafer. However, increasing the mass of the chuck results in increased force necessary to move the chuck, and consequently increases stage mass and motor power levels.
Many of the manufacturing steps used to create integrated circuits, and other small dimension devices on substrates, require that the wafer or substrate position and orientation be precisely controlled. This requirement may be meet using what is known as an X Y stage to manipulate the wafer over a planar region, and what is known as a Z theta chuck system to raise and lower and rotate the wafer about an axis normal to the nominal XY plane. Certain processes also require active manipulation of the plane of the wafer in order to maintain the wafer surface parallel to the plane of the process tool. This may be necessary if the wafer front surface and the wafer back surface are not exactly parallel. This condition is known as taper. The thickness of the wafer may also vary from place to place, a situation known as bow. Thus, a chuck may be required to rotate the front surface of the wafer in what is known as roll, pitch and yaw. Each of these three motions can be considered to be rotations around the X, Y or Z axis respectively.
Processes that require these sorts of motions include step and repeat camera imaging systems, which need the front surface of the wafer to be flat over a large surface area. If a change in the front surface location with respect to the focal plane of the camera occurs during any of the rotations around the orthogonal axes, then the resulting image will not be in focus at all points.
As semiconductor technology has increased with improved semiconductor performance, the wafer diameters have increased over successive generations of semiconductor manufacturing equipment from less than 100 mm to the current standard of 300 mm. At the same time, the precision requirements of the semiconductor manufacturing equipment has become tighter as the critical line width sizes have become smaller with the increased technological level. The requirement to maintain tighter line widths that accompanies the increase in technological level, also results in increased alignment accuracy and precision requirements, and to a decreased depth of field capability. The depth of field problem requires that the wafer surface be flatter, which consequently requires that the wafer chuck be flatter and strong enough to hold the wafer flat. The increase in required precision thus includes the need for improved capabilities to move the wafer accurately in the horizontal plane, the XY plane, as well as in the vertical direction, i.e. Z. Increase in required precision also requires accurate motion of the wafer in the roll, pitch and yaw directions.
Traditional chuck systems rely on mechanical bearings and machining tolerances to maintain the plane of the wafer attached to the chuck parallel to the XY plane of the stage. Mechanical approaches to a wafer chuck become more difficult as the mass of the chuck increases and the precision requirements become more severe. This is because as the wafer chuck mass increases, the mechanical bearings used to constrain the chuck necessarily become larger. As the precision requirements increase, the mechanical bearings must resort to increased levels of what is known as a preload in order to achieve the necessary stiffness to maintain precision and avoid vibration. As each of the elements becomes more massive and the stiffness increases, the forces required to support the more massive chuck and overcome the friction of bearings and actuators also consequently increases. Typical electromechanical actuators, such as motors, dissipate power in proportion to the square of the force they produce. Thus, as the chuck mass increases and the bearing mass increases, the size of the actuator and the actuator power must also increase, resulting in increased power dissipation and local heating of the wafer. Heating of the wafer may be a problem because expansion of the wafer results in a shifting of the location of different parts of the wafer, and thus loss of precision and repeatability. The different coefficients of thermal expansion of the aluminum (or other material) chuck and the semiconductor wafer may also result in mismatched stress between the chuck and the wafer, and may result in wafer warpage. Thus, power dissipation in the actuators of the chuck may lead to thermal gradients and corresponding changes in the mechanical dimensions of the chuck mechanism, which may be another major impediment to achieving the levels of precision required in many semiconductor processes, liquid crystal display processes, thin film magnetic head processes, and micro-machining processes.
Some of the above described problems have been addressed in prior art chuck mechanisms by restricting the range of Z motion to less than 0.11 mm, and using a flexure suspended chuck driven in the Z direction by piezo actuators. While these devices provide large forces with negligible heat generation, they are unable to provide sufficient range of motion to allow wafer transfers between a transfer robot and the chuck. This is because during the loading and unloading of a wafer onto the chuck, a minimum gap of approximately 6 mm must be established between the bottom of the wafer and the top of the chuck. This spacing is necessary for the robot or operator to insert a vacuum paddle between the chuck and the wafer to move the wafer while only touching the wafer backside, and thus prevent damage to the front surface of the wafer. Contact with the front surface of the wafer may result in physical damage such as scratching, and may also result in contamination of the devices on the front surface. Thus, piezo actuators must have a separate mechanism to provide the chuck with enough separation to allow wafer transfers. This additional requirement of piezo actuators increases the cost, complexity, and the mass of the stage.
Another problem with mechanical methods of moving a wafer chuck around, such as the piezo actuators, is that the physical contact of the piezo actuators with the chuck may represent another source of stress and strain, and therefore cause deformation of the chuck and resulting loss of precision. As noted before, certain manufacturing processes, such as semiconductor device manufacture, liquid crystal display manufacture, and thin film magnetic head manufacture, require extreme flatness in the manufactured device, and consequently extreme flatness in the chuck.
Since the chuck mechanism is often carried on an XY stage, the chuck design can have a major influence on the XY stage design and performance. The XY stage performance is affected by the mass and height of the chuck. As the chuck mass increases, the stage must be made larger and must be able to dissipate more power and heat in order to achieve a sufficient level of performance and precision. Since the power dissipation increases with the square of the total stage mass, a small increase in mass may result in a large increase in power consumption. The heat dissipated by the stage influences the stage accuracy and precision due to thermal disturbance of the air inside a tool, since the thermal gradients disrupt the precision of the distance measuring laser interferometers that are typically used to measure the location of the stage. Changes in heat dissipation also affect the precision of the interferometer, and thus the repeatability of the operation. As an example, the operation of a step and repeat camera requires the movement of the wafer chuck from image field to image field be as precise as possible in order to minimize the amount of time spent on each image field in fine alignment.
Many semiconductor process tools involve optical lens elements that must be in close proximity to the top of the wafer surface. The clearance between the top of the wafer surface and the process tools, for example in a step and repeat camera, or a direct write laser beam lithography tool, may be less than 1 mm. In the specific case of wafer probing, actual mechanical contact is made between the wafer and probe pins. Thus, in order to achieve the necessary clearance for wafer transfers, i.e.  greater than 6 mm, the wafer must be raised or the chuck must be lowered. Since the separation of wafer and processing tool maybe less than 1 mm, in many such cases the wafer cannot be raised, and thus the chuck must be lowered. However it has been previously noted that it is difficult to have high precision and large motion simultaneously. Thus prior art chuck systems have the XY stage carrying the chuck move away from underneath the tool before wafer transfers occur. This causes increased time, cost, size and heat dissipation of the stage.
The trend in the various fabrication industries, such as semiconductor, liquid crystal display, thin film magnetic head, and micro-machining, toward larger substrate sizes and increased precision in chuck location, results in the need for a chuck that can provide precision control in X, Y, Z, roll, pitch, and yaw. The chuck must also provide a large motion in the Z direction, minimize the mass and power needed to move the chuck on the XY stage, maintain a constant temperature at the chuck and wafer, and not apply stress to the wafer. The chuck must also be able to compensate for externally applied forces on the wafer, such as found in wafer probers.
According to the present invention, an apparatus for manipulation of a planar substrate includes a housing movable on an X-Y stage that transports the housing in at least two orthogonal directions, a surface handler disposed within the housing and having a substantially flat surface, a plurality of position sensors disposed about at least one of a periphery of the surface handler and a periphery of the housing, at least one air-bearing sleeve disposed in the housing, at least one piston disposed within the at least one sleeve and having at least one pressure chamber formed by the piston and air-bearing sleeve, at least one valve that may be used to modulate the flow of fluid from the pressure chamber to an exhaust pressure, at least one voice coil motor disposed within the piston, at least one air-bearing pad disposed at one end of the piston opposite the pressure chamber and acting against a surface of the surface handler opposite the flat surface where the pressure from the air-bearing pad against the surface handler is opposed by a magnetic attraction between the surface handler and the piston, a plurality of magnetic regions located at preselected portions of the surface handler, a plurality of radial actuators disposed on the housing and corresponding to at least a portion of the plurality of magnetic regions of the surface handler, and a plurality of tangential actuators that correspond to at least a portion of the magnetic regions of the surface handler.
The housing may move the surface handler in at least an X direction, a Y direction, a Z direction, a yaw direction, a roll direction and a pitch direction. The housing may include at least one actuator for each direction of motion. The planar surface handler may be completely supported and transported by the actuators. The surface handler may be a chuck. The chuck may be a vacuum chuck. The chuck may be an anodized aluminum alloy circular cylinder. The chuck may have a central region having an array of grooves embedded in the flat surface and connected to a vacuum control line. The central region may be a circle having a radius of approximately 200 mm, and the array of grooves may be a plurality of concentric circular grooves having a common center at approximately a center of the chuck. The chuck may further include a peripheral region having a second array of grooves embedded in the flat surface and connected to a second vacuum control line. The second array of grooves may be an annular region of approximately 200 to 300 mm radius. The substantially flat surface of the surface handler may include a plurality of vacuum conduits. The plurality of vacuum conduits may include at least two different vacuum control areas, each area having a separately controllable vacuum pressure source. The plurality of vacuum conduits may be arranged in rings. The surface handler may not directly contact the housing. The plurality of position sensors may be three. The plurality of position sensors may provide position measurements in two dimensions. The surface handler may include a plurality of holes through which pins, fixedly attached to the housing, are inserted perpendicular to the flat surface of the surface handler, and, in response to a signal that lowers the surface handler with respect to the housing, the pins may extend above the flat surface by at least a predetermined distance, which may be greater than approximately 6 mm. The plurality of holes may be three. The plurality of pins may be hollow. The hollow pins may be connected to a vacuum line. The surface handler may have an approximately circular top surface. The magnet of the voice coil motor disposed within the piston may provide the source of magnetic flux that preloads the surface handler against an air bearing pad. The magnet may attract the ferromagnetic region of the surface handler against the air bearing with a force sufficient to establish a desired stiffness and corresponding flying-height between the surface handler and the air-bearing pad disposed on top of the piston. The piston may include an air-bearing pad disposed above the surface of the piston and allowed to pivot with respect to a vertical axis of the piston. The pivot may be a ball pivot. The fluid in the sleeve may be air. The apparatus may also include a controller that modulates the valve to regulate the pressure in the pressure chamber and modulates the force in the voice coil motor in order to adjust the height of the surface handler with respect to the housing. The piston may have no sealing surface with respect to the sleeve to prevent the compressed air from leaking out at a controlled rate. The controlled rate may be determined by relative diameters of the piston and the sleeve. The apparatus may also include a single valve connected to each pressure chamber and normally operated in a partially open state to regulate the flow of fluid from the pressure chamber to an external source of pressure that is maintained at a pressure substantially below the nominal chamber pressure. The valve may be opened or closed in proportion to a signal from the controller in order to regulate the net mass flow of fluid between the fluid entering the chamber from the air-bearing sleeve and the mass flow of fluid leaving the chamber through the valve. The plurality of pistons may be three. The pistons may be driven differentially to generate motion around the X and Y axes. The controller may operate the valve and corresponding voice coil motor so as to minimize the power dissipated in each voice coil motor. The plurality of magnetic regions of the surface handler may be three. The plurality of tangential actuators may be three. The coils of the tangential actuators may be disposed in a portion of the housing and the low-reluctance components of the actuators may be fixed in the surface handler. The surface handler may have a plurality of ferromagnetic surfaces disposed around a peripheral portion of the surface handler that interact with a plurality of radially-acting electromagnetic actuators disposed in the housing. The plurality of actuators may be three. The surface handler may further include a plurality of projections disposed in a symmetric pattern on the surface opposite the flat surface and wherein the housing includes a plurality of depressions disposed in a corresponding pattern. The plurality of projections may equal three. The plurality of projections may further include balls and the plurality of depressions include vee-grooves. The apparatus may further include a control system that controls the plurality of radial actuators in a differential mode where the total amount of power provided to all of the radial actuators is maintained at a substantially constant value while generating net forces between the housing and surface handler in the X and Y plane. The housing may further include a recessed bottom portion disposed to fit into a recess in a corresponding X-Y stage.
According further to the present invention, an apparatus for manipulation of a substrate includes a handler having a flat surface that holds a substrate, a housing that holds the handler, a plurality of tangential actuators that cause rotation of the handler about an axis perpendicular to the surface of the handler, and a plurality of radial actuators, each actuator moving the handler with respect to the housing in a selected radial direction independent of rotation caused by the tangential actuators, wherein an increase in the force level of one of the plurality of radial actuators is counterbalanced by a reduction in the force level of at least one other of the radial actuators so that the power consumed by all of the radial actuators together is substantially constant.
According further to the present invention, an apparatus for manipulation of a substrate includes a handler that holds a substrate, a housing having a bottom surface that holds the handler, and a plurality of vertical actuators disposed in the housing to move the handler toward and away from the bottom surface of the housing, wherein an increase in distance from the bottom surface to the handler provided by one of the vertical actuators is counterbalanced by a decrease in distance from the handler to the bottom surface by another one of the vertical actuators to provide a rotation about an axis of rotation perpendicular to a direction of motion provided by the vertical actuators, wherein the axis of rotation is disposed at a location corresponding to a location in a substrate provided on the handler. The vertical actuators may include a magnet that attracts a magnetic plate disposed on the bottom surface of the handler and an air bearing that pushes against the plate. The handler may rotate in a plane that is substantially perpendicular to the direction of motion provided by the vertical actuators.
According further to the present invention, an apparatus for manipulation of a substrate includes a handler that holds the substrate, a housing that holds the handler, a plurality of patterned surfaces disposed on one of: the housing and the handler, wherein the patterned surfaces include one of the following combinations: at least six linear arrays, at least four linear arrays and at least one grid array, at least two linear arrays and at least two grid arrays, or at least three grid arrays, and a plurality of readers that read the combination to determine a location of the substrate in six degrees of freedom. The readers may be optical encoders.
According further to the present invention, an apparatus for manipulation of a substrate includes a handler that holds the substrate, a housing that holds the handler, and plurality of actuators that move the handler in response to at least one control signal, here the plurality of actuators further includes at least one high frequency actuator and at least one low frequency actuator that provide motion in a single direction and wherein the at least one control signal includes a high frequency portion that is provided to the at least one high frequency actuator and a low frequency portion that is provided to the at least one low frequency actuator. The at least one low frequency actuator may reduce static forces on the at least one high frequency actuator. The at least one low frequency actuators may include at least one radial electromagnet. The at least one high frequency actuator may include at least one tangential electromagnet. The at least one low frequency actuator may be a pneumatic actuator. The pneumatic actuator may actuate the handler in a vertical direction. The at least one high frequency actuator may include an electromagnetic actuator that provides a force in a vertical direction on the handler.
According further to the present invention, an apparatus for manipulation of a substrate includes a handler that holds the substrate, a housing that holds the handler, and a plurality of vertical actuators disposed in the housing to move the handler toward and away from the bottom surface of the housing, wherein the vertical actuators are driven differentially to counteract and compensate for any vertical off axis forces applied to the substrate. The vertical actuators may include a magnet that attracts a magnetic plate disposed on a bottom surface of the handler and an air bearing that pushes against the late. The apparatus may further include a ball disposed in each vertical actuator and a corresponding vee-groove disposed in the handler to allow roll and pitch motion of the handler. Compensation for vertical off axis forces may be provided primarily by a pneumatic actuator.
According further to the present invention, an actuator includes a piston movably guided by a fluid-bearing, a fluid chamber formed at one end of the piston having a chamber pressure controlled by balancing a fluid flowing into the fluid chamber with fluid exiting the chamber through a controllable orifice, and a voice coil motor disposed to move the piston located within the confines of the piston. The actuator may also include a control system that supplies large low-frequency forces by modulating the chamber pressure, and supplies low amplitude high-frequency forces via the voice coil motor.
According further to the present invention, an actuator that manipulates an object includes at least three tangentially acting voice coil motors disposed to move the object relative to the housing in a yaw direction, at least three radially acting electromagnet actuators maintaining a substantially constant gap between the object and a housing independent of motion in the yaw direction, and a control system that supplies the object with large low-frequency forces in a plane of the object using the radially acting electromagnet actuators, and supplies low amplitude high-frequency forces in the plane of the object and provides forces for motion in the yaw direction using the tangential voice coil motors.
According further to the present invention, a substrate manipulator providing six degrees of controllable motion for an object includes a housing, a substrate handler that holds the object and does not contact the housing, at least three actuators that move the object in Z, Roll and Pitch directions, and at least three actuators that move the object in X, Y and Yaw directions.
According further to the present invention, a substrate manipulator providing control of movements of an object in the Z, roll and pitch directions includes a housing, a substrate handler that holds the object disposed in the housing, and at least three actuators that move the object in a Z direction.
According further to the present invention, a substrate manipulator providing control of movements in the Z, roll, pitch and yaw directions, includes a housing, a surface handler disposed in the housing, at least one hub, disposed concentric to a center point of the housing and free to rotate about the Z axis using at least three magnetically preloaded air bearing regions located between the housing and the hub, a pair of magnetically preloaded bearings that constrain the hub to rotate substantially about the axis of the housing, at least three tangential voice coil motors that provide a yaw moment without creating any substantial XY force on the hub relative to the housing, and at least three Z actuators to move the surface handler with respect to the hub.
The size and configuration of the pneumatic actuator may be selected to enhance actuator performance in precision applications. The diameter of the air bearing sleeve affects the vertical actuator performance in three aspects.
The stiffness of the air bearing to piston assembly is influenced by the operating pressure and circumference of the air-bearing sleeve. A larger sleeve will yield greater stiffness for the same fluid pressure supplied to the air bearing. Greater stiffness is preferred in order to enhance precision of the manipulator.
A larger diameter air bearing sleeve implies a larger effective area of the pneumatic piston. For substantially similar net vertical force a larger piston area will require lower pressures in the pressure chamber. Generally one avoids low pressures in a pressure control situation due to the somewhat larger valve opening that is required to achieve a desired mass flow rate. In the present invention, the low chamber pressure is an advantage in that it effectively isolates the chamber pressure from the air-bearing performance. For an example cylinder diameter of approximately 50 mm, a typical working pressure in the pressure chamber is between 0 and 0.05 bar (that is between 0 and 5% of 1 atmosphere) above atmospheric pressure. Such low operating pressures ensure that the air bearing is unaffected by the minute changes in chamber pressure experienced during normal operations. The degree of effective isolation between the chamber and the air bearing ensures that the mass flow of fluid into the chamber is nearly constant in normal operation.
The low chamber pressure and therefore constant mass flow of fluid into the chamber from the air bearing provide an extremely xe2x80x9cquietxe2x80x9d source of chamber pressure. A well known problem with fluid control systems is the level of noise that is often injected in to the pressure chamber due to turbulence in the servo valve. This is particularly true when a servo valve is used to control the rate of fluid entering the chamber. As the fluid passes across the control orifice in the valve, the fluid may enter the turbulent regime (Reynolds number above 2000). The turbulence in the fluid creates minute pressure disturbances that may be apparent as a source of noise making nanometer-level of precision difficult to achieve. The present invention avoids the use of an xe2x80x9corificexe2x80x9d sourcing fluid into the chamber. The air gap between the piston and air bearing sleeve are sufficient to ensure that the fluid entering the pressure chamber exhibits laminar flow. This avoids the xe2x80x9cnoisexe2x80x9d of turbulent flow. The servo valve placed on the exhaust with an exhaust pressure to a pressure xe2x80x9csourcexe2x80x9d held substantially below the pressure in the chamber (ie: a source of vacuum) ensures that the exhaust valve will be operated in the turbulent regime. Since the turbulence is on the exhaust side of the pressure chamber, the xe2x80x9cnoisexe2x80x9d associated with the turbulent flow in the exhaust valve does not influence the pressure chamber. Furthermore, it will be readily apparent to those skilled in the art of compressible fluid flow, that operating the exhaust valve in the turbulent regime (above Reynolds number of 2000) ensures that no pressure variations in the exhaust pressure (ie: variation in the vacuum pressure) will propagate across the servo valve and influence either the chamber pressure or the mass flow through the servo valve. This allows a less expensive vacuum pump to be used.